In a steering apparatus for a motor vehicle in recent days, a motor-driven power steering apparatus making good use of an electric motor is well employed as a steering assist mechanism. The motor-driven power steering apparatus transmits a rotating torque of the electric motor to a steering shaft via a worm gear.
However, in the worm gear, since a transmission efficiency of the rotating torque is comparatively low such as 60 to 80%, in the case that a speed reduction ratio is set constant, an electric motor having a larger output torque is necessary for transmitting a predetermined rotating torque. Accordingly, there is a problem that an outer diameter of the electric motor becomes large and it is hard to make an entire of the steering apparatus compact. Therefore, there has been designed a speed reducer in which the output shaft of the electric motor is attached in such a manner as to be in almost parallel to the steering shaft, and which uses a spur gear or a helical gear in which the transmission efficiency of the rotating torque is comparatively high.
In the case of using the spur gear or the helical gear for the speed reducer, the transmission efficiency of the rotating torque becomes comparatively high such as about 95%. Accordingly, it is possible to reduce the output torque of the electric motor at that degree, and it is possible to make the entire of the steering apparatus compact by suppressing an enlargement of an outer shape of the electric motor.
However, in the case of employing the speed reducer, for example, using the spur gear, if it is intended to obtain a necessary speed reduction ratio on the basis of a one-stage structure of the gear provided in the output shaft of the electric motor, and the gear attached to the steering shaft engaging with the gear, a pitch circle of the steering shaft side gear becomes large, and it is impossible to improve the condition that it is hard to intend to make the entire of the steering apparatus compact.
On the other hand, in the case of employing a speed reducer having a multi-stage structure, for example, having intermediate gears, in place of the speed reducer having the one-stage structure, as the speed reducer using the spur gear, it is possible to intend to make the entire of the steering apparatus compact, however, there is generated a new problem that a comfortable steering feeling is reduced due to an increase of a backlash, a cost increase due to a complication of the structure of the speed reducer or the like.
In order to solve the problem mentioned above, for example, Japanese Patent Application Laid-Open No. 11-124045 discloses a motor-driven power steering apparatus in which an entire of a steering apparatus having an electric motor and a speed reducer arranged therein can be made compact by accommodating a speed reducer constituted by a pair of spur gears or helical gears having a high speed reduction ratio within a housing, and arranging the electric motor close to the housing in which a steering shaft is accommodated.
In the motor-driven power steering apparatus disclosed in the Japanese Patent Application Laid-Open No. 11-124045, since it is hard to secure a strength of the gear on the basis of a normal involute tooth profile, a strength of a tooth surface is secured by employing a tooth profile based on a predetermined special theory.